Particle or photon counting or particle-counting techniques are well-known, e.g. in nuclear physics, astronomy, medical imaging, security, etc. These techniques are usable for detection and measurement of high-energy photons or particles of X-rays, Gamma-rays and ionized particles, etc. A detection system based on detector devices and dedicated readout circuits is used to amplify the photocurrent or photovoltage of the detectors, shape them, and make them ready for discrimination by analog-to-digital converters, or by comparing the pulse amplitude with known threshold levels.
Within high-quality image capturing, such as in medical imaging by computer tomography, a very high photon count rate is required to provide a wide dynamic range in the images. In spite of high-speed digital processing equipment, an inhibiting factor in achieving a high dynamic range is the problem that particles captured by a detector provide an electric pulse having a considerable width, i.e. a considerable temporal extension. Thus, a particle may be captured by the detector when a particle pulse captured earlier is still within its decay time, and, consequently, the particle pulse captured later may not be detected at all and thus has a limit for the maximally detectable particle rate, thereby also limiting the available dynamic range for the subsequent image-processing operation. The reason is that it is difficult for the subsequent counting circuits to discriminate such particles captured closely spaced in time, since in the electric signal the pulse from the particle captured later may more or less overlap or mask the pulse from the earlier particle. This problem is known as the pile-up effect.
The pile-up problem is addressed, for example, in GB 2 332 513 A which discloses a nuclear spectroscopy system in which pile-up is detected. A pulse length compensation, i.e. shortening or lengthening the pulse shortening or pulse widening, can then be performed with the object of keeping the pulse shape constant. The method described in GB 2 332 513 A solves the problem of providing a constant pulse shape also in the case of pile-up. However, the pile-up problem itself is not solved, and, consequently, the count rate is not improved.